This invention relates to the field of electromagnetic radiators, specifically transverse electromagnetic (TEM) horn antennas.
TEM horn antennas employ the transverse electromagnetic mode of operation, which means that both the electric field and the magnetic field are transverse (perpendicular) to the direction of wave propagation in or along the antenna structure. TEM horn antennas have a well defined time-domain behavior as a two-conductor smoothly expanding traveling-wave structure. As a transmitting antenna, a TEM horn guides the desired electromagnetic wave from the feedpoint to the aperture, where the wave then breaks away and radiates into space, forming an endfire radiation pattern. As a receiving antenna, a TEM horn intercepts an incident electromagnetic wave and generates an induced current which can then be measured by an instrument connected to the feedpoint.
Because of the well-behaved traveling-wave behavior of TEM horns, they offer very fast transient (time-domain) response, and correspondingly ultra-wideband frequency-domain operation (with low phase dispersion). This phase response is much less dispersive than other types of horn antennas, which utilize a single conductor waveguide topology. A single conductor waveguide structure prohibits a TEM mode of operation, limiting the electromagnetic operation to either transverse electric modes, transverse magnetic modes, or a hybrid combination of both. The most non-dispersive and frequency-independent electromagnetic operation is achieved with the TEM mode, meaning that TEM horn antennas offer the best wideband performance of horn antennas. This makes them attractive in communications or radar systems which are ultra-wideband in nature (i.e., multiple octaves or decades of bandwidth). The wideband operation provides increased information content, meaning higher data rates in communications applications and increased (or finer) range resolution in radar applications.
TEM horn antennas typically incorporate long flat triangularly shaped conducting plates which separate or spread apart at some constant angle. However, they may employ exponential (or other shaped) flaring in plate width or plate separation or both. Examples of the constant taper type include McCorkle, U.S. Pat. Nos. 5,471,223 and 5,606,331, and Heger et al., U.S. Pat. No. 5,640,168. Examples of the flared type include Brillouin, U.S. Pat. No. 2,454,766, Wichmann, U.S. Pat. No. 4,811,027, and Cermignani et al., U.S. Pat. No. 5,325,105. Both types are included in Carr, U.S. Pat. No. 3,099,836, and Podgorski and Gibson, U.S. Pat. No. 5,440,316. The exact transverse shaping of the antenna plates (including their cross-sectional shape) and their separation along the length of the antenna determine both the characteristic impedance (or surge impedance) variation along the structure and also the radiation properties of the antenna.
In addition, TEM horn antennas can incorporate aperture lenses to collimate the spherical wavefront inside the two conductor expanding structure, in order to improve the transient response and higher frequency gain characteristic. See, e.g., Carr, U.S. Pat. No. 3,099,836.
TEM horn antennas can also employ resistive loading in order to reduce the ringing of the antenna currents up and down the structure. This resistive loading may be incorporated in one of two different manners: either continuously distributed along each of the two conductive plates (from the feedpoint out to the aperture on both the interior and exterior surfaces), or located at the aperture end of the conductive plates (as either parallel plate extensions or discrete resistive terminations of the aperture currents). The aperture-connected resistive loading implementation is described in Wichmann, U.S. Pat. No. 4,811,027, Podgorski and Gibson, U.S. Pat. No. 5,440,316, and McCorkle, U.S. Pat. Nos. 5,471,223 and 5,606,331. In all cases the express intent and purpose of the resistive loading has been to attenuate the outgoing traveling currents on the conducting plates in order to reduce the subsequent ringing of the TEM horn.
Unfortunately, none of the above variations of TEM horn antennas have addressed a fundamental problem in the inherent design of the TEM horn structure: the existence of oppositely polarized electromagnetic fields on the exterior surfaces of the antenna plates, and the consequent reduction in main-beam transmit gain and receive sensitivity due to partial field cancellation of the primary interior electromagnetic wave supported in between the antenna plates.
Consequently, the need exists for a TEM horn design which directly addresses the fundamental problem of oppositely directed electromagnetic fields and currents on the exterior surfaces of the antenna plates, in order to achieve an inherently more efficient radiating antenna structure due to the reduction or elimination of these exterior fields.